As a piezoelectric material a ceramic material of PZT (PbZrO3—PbTiO3 type solid solution) has been heretofore broadly used, however, since PZT contains lead, a polymeric piezoelectric material, which imposes less environmental burden and has higher flexibility, has been coming into use.
Currently known polymeric piezoelectric materials can be classified roughly into 2 types. Namely, 2 types of poled polymers, as represented by nylon 11, polyvinyl fluoride, polyvinyl chloride, polyurea, etc. and ferroelectric polymers, as represented by (β-type) polyvinylidene fluoride (PVDF), a vinylidene fluoride-trifluoroethylene copolymer (P(VDF-TrFE)) (75/25), etc. However, polymeric piezoelectric materials are inferior to PZT in terms of piezoelectricity, and therefore improvement of the piezoelectricity has been demanded. In order to cope with the above, attempts at improvement of the piezoelectricity of polymeric piezoelectric materials have been made from various viewpoints.
For example, PVDF and P(VDF-TrFE), which are ferroelectric polymers, have superior piezoelectricity among polymers and a piezoelectric constant d31 of 20 pC/N or higher. A film material formed from PVDF or P(VDF-TrFE) is imparted with piezoelectricity by orientating polymer chains by a stretching operation in the stretching direction; then building up opposite electric charges on the back and front sides of the film by means of corona discharge, etc. to generate an electric field perpendicular to the film surface and to orientate permanent dipoles containing fluorine in side chains of the polymer chains parallel to the electric field. However, there has been a problem in view of practical use that the orientation of permanent dipoles achieved by a poling treatment tends to relax, because an opposite electric charge of water or an ion in the air can easily attach to a polarized film surface in the direction of canceling the orientation, and the piezoelectricity declines remarkably with time.
Although PVDF is a material that exhibits the highest piezoelectricity among the above described polymeric piezoelectric materials, its dielectric constant is 13 and relatively high among polymeric piezoelectric materials, and therefore the piezoelectric g constant (open circuit voltage per unit stress), which is a value obtained by dividing a piezoelectric d constant by a dielectric constant, becomes small. In addition, although PVDF exhibits favorable conversion efficiency from electricity to sound, improvement in the conversion efficiency from sound to electricity has been looked for.
In recent years, use of an aliphatic polyester having optical activity, such as polylactic acid, has drawn attention in addition to the above polymeric piezoelectric materials. A polylactic acid type polymer is known to exhibit piezoelectricity by a simple mechanical stretching operation.
Among polymers having optical activity, the piezoelectricity of a polymer crystal, such as polylactic acid, results from permanent dipoles of C═O bonds existing in the screw axis direction. Especially, polylactic acid, in which the volume fraction of side chains with respect to a main chain is small and the content of permanent dipoles per volume is large, thereby constituting a sort of ideal polymer among polymers having helical chirality.
Polylactic acid exhibiting piezoelectricity only by a stretching treatment does not require a poling treatment and is known to maintain the piezoelectric modulus without decrease for several years.
Since polylactic acid exhibits various piezoelectric properties as described above, various polymeric piezoelectric materials using polylactic acid have been reported.
For example, a polymeric piezoelectric material exhibiting a piezoelectric modulus of approximately 10 pC/N at normal temperature, which is attained by a stretching treatment of a molding of polylactic acid, has been disclosed (e.g., see Japanese Patent Application Laid-Open (JP-A) No. 5-152638).
Further, it has been also reported that high piezoelectricity of approximately 18 pC/N can be achieved by a special orientation method called as a forging process for orientating highly polylactic acid crystals (e.g., see JP-A-2005-213376).